+2.5V to +5.5V RS-232 Transceivers

in UCSP

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7

For example, a three-cell NiMh battery system starts at

output voltage of ±5.5V. As the battery discharges, the

MAX3228/MAX3229 maintain the outputs in regulation

until the battery voltage drops below +3.1V. Then the

output regulation points change to ±4.0V

age of +3.5V, the charge pump switches modes to

generate an output of ±5.5V.

Table 1 shows different supply schemes and their

operating voltage ranges.

RS-232 Transmitters

The transmitters are inverting level translators that con-

vert CMOS-logic levels to RS-232 levels. The

MAX3228/MAX3229 will automatically reduce the RS-

232 compliant levels (±5.5V) to RS-232 compatible lev-

The reduced levels also reduce supply current require-

ments, extending battery life. Built-in hysteresis of

sudden current draw causing the supply voltage to

drop slightly. The outputs will return to RS-232 compli-

+3.5V.

The MAX3228/MAX3229 transmitters guarantee a

250kbps data rate with worst-case loads of 3k

Ω in par-

allel with 1000pF.

When FORCEOFF is driven to ground, the transmitters

and receivers are disabled and the outputs become

high impedance. When the AutoShutdown circuitry

senses that all receiver and transmitter inputs are inac-

tive for more than 30µs, the transmitters are disabled

and the outputs go to a high-impedance state. When

the power is off, the MAX3228/MAX3229 permit the

transmitter outputs to be driven up to ±12V.

The transmitter inputs do not have pullup resistors.

RS-232 Receivers

The MAX3228/MAX3229 receivers convert RS-232 sig-

nals to logic output levels. All receivers have inverting

three-state outputs and can be active or inactive. In

shutdown (FORCEOFF = low) or in AutoShutdown, the

MAX3228/MAX3229 receivers are in a high-impedance

state (Table 3).

The MAX3228/MAX3229 feature an INVALID output that

is enabled low when no valid RS-232 signal levels have

been detected on any receiver inputs. INVALID is func-

tional in any mode (Figures 2 and 3).

Figure 2c. MAX322_ AutoShutdown Logic

FORCEOFF

POWER DOWN

INVALID

FORCEON

INVALID IS AN INTERNALLY GENERATED SIGNAL

THAT IS USED BY THE AUTOSHUTDOWN LOGIC

AND APPEARS AS AN OUTPUT OF THE DEVICE.

POWER DOWN IS ONLY AN INTERNAL SIGNAL.

IT CONTROLS THE OPERATIONAL STATUS OF

THE TRANSMITTERS AND THE POWER SUPPLIES.

Figure 2a. MAX322_ Entering 1µA Supply Mode via

AutoShutdown

+0.3V

-0.3V

INVALID

TO MAX322 _

POWER SUPPLY

AND TRANSMITTERS

R_IN

*TRANSMITTERS ARE DISABLED, REDUCING SUPPLY CURRENT TO 1

µA IF

ALL RECEIVER INPUTS ARE BETWEEN +0.3V AND -0.3V FOR AT LEAST 30

µs.

30

µs

COUNTER

R

Figure 2b. MAX322_ with Transmitters Enabled Using

AutoShutdown

+2.7V

-2.7V

INVALID

TO MAX322 _

POWER SUPPLY

R_IN

*TRANSMITTERS ARE ENABLED IF:

ANY RECEIVER INPUT IS GREATER THAN +2.7V OR LESS THAN -2.7V.

ANY RECEIVER INPUT HAS BEEN BETWEEN +0.3V AND -0.3V FOR LESS THAN 30

µs.

30

µs

COUNTER

R